Biodegradable polyhydroxyalkanoate copolymers having improved crystallization properties

ABSTRACT

Compositions having improved crystallization properties and physical properties including a continuous phase of a first biodegradable polyhydroxyalkanoate comprising a copolymer, or a blend thereof, of at least two randomly repeating monomer units and having a melting point Tm1, and a second crystallizable biodegradable polyhydroxyalkanoate homopolymer or copolymer, which comprises at least one randomly repeating monomer unit and has a melting point Tm2, where Tm2 is at least about 20□C. greater than Tm1. The compositions are formed by solution or melt blending of the components and may be formed into shaped articles.

CROSS-REFERENCE

This application is a continuation of International ApplicationPCT/US01/50462, with an international filing date of Dec. 20, 2001,which claims benefit of Provisional Application Ser. No. 60/257,911,filed Dec. 21, 2000.

FIELD OF THE INVENTION

The present invention is directed to biodegradable semicrystallinepolyhydroxyalkanoate copolymers and blends containing such copolymershaving improved crystallization properties, to methods for improving thecrystallization rates and physical properties of such semicrystallinecopolymers, to methods of forming shaped articles from such copolymers,and to shaped articles formed by such methods.

Shaped articles formed with such copolymers include, but are not limitedto, films, fibers, nonwovens, sheets, membranes, coatings, binders,foams and molded products for packaging. The products exhibit adesirable combination of high crystallization rate, ductility andflexibility, and importantly biodegradability. Additional benefits ofsuch blends are described in the invention. The products are useful fora variety of biodegradable articles, such as diaper topsheets, diaperbacksheets, disposable wipes, shopping and lawn/leaf bags, agriculturalfilms, yard waste nets, fishing nets, seeding templates, flower pots,disposable garments, medical disposables, paper coatings, biodegradablepackaging, binders for cellulose fibers or synthetics, and the like.

BACKGROUND OF THE INVENTION

This invention relates to the need for alleviating the growingenvironmental problem of excessive plastic waste that makes up an evermore important volume fraction of what get thrown out in landfills everyyear. Biodegradable polymers and products formed from biodegradablepolymers are becoming increasingly important in view of the desire toreduce the volume of solid waste materials generated by consumers eachyear. The invention further relates to the need for developing newplastic materials that can be used in applications wherebiodegradability, compostability or biocompatibility, are among primarydesirable features of such applications. Such examples include forinstance agricultural films, and the convenience that such films offerto farmers when they do not have to be collected after they have servedtheir purpose. Flower pots or seeding templates are other examples wherethe temporary nature of the substrate translates into convenience forthe user. Similarly, means of disposal of sanitary garments, such asfacial wipes, sanitary napkins, pantiliners, or even diapers, may alsobe advantageously broadened with the use of materials that degrade inthe sewage. Such items could be easily disposed directly in the sewage,after use, without disrupting current infrastructure (septic tanks orpublic sewage), and giving the consumer more disposal options. Currentplastics typically used in making such sanitary garments can not bedisposed without undesirable material accumulation. New materials to beused in the examples above would ideally need to exhibit many of thephysical characteristics of conventional polyolefins; they must be waterimpermeable, tough, strong, yet soft, flexible, rattle-free, possiblylow-cost and must be capable of being produced on standard polymerprocessing equipment in order to be affordable.

Another application which illustrates the direct benefit of compostablethermoplastic materials are leaf/lawn bags. Today's sole compostable bagwhich does not require the composter the additional burden of bagremoval and the risk of compost contamination is the paper bag. Yet, itfails to provide the flexibility, the toughness and moisture-resistanceof plastic films, and is more voluminous to store. Compostable plasticfilms used to make leaf/lawn bags would provide bags that could bedisposed much like paper bags, yet provide the convenience of plasticbags.

It becomes clear in view of these examples that a combination ofbiodegradability, melt-processability and end-use performance is ofparticular interest to the development of a new class of polymers. Meltprocessability is key in allowing the material to be converted in films,coatings, nonwovens or molded objects by conventional processingmethods. These methods include cast film and blown film extrusion ofsingle layer structures, cast or blown film co-extrusion ofmulti-layer-structures. Other suitable film processing methods includeextrusion coating of one material on one or both sides of a compostablesubstrate such as another film, a non-woven fabric or a paper web. Otherprocessing methods include traditional means of making fibers ornonwovens (melt blown, spun bounded, flash spinning), and injection orblow molding of bottles or pots. Polymer properties are essential notonly in ensuring optimal product performance (flexibility, strength,ductility, toughness, thermal softening point and moisture resistance)during end-use, but also in the actual product-making stages to ensurecontinuous operations. Rapid crystallization of the processed polymermelt upon cooling is clearly an essential feature necessary for thesuccess of many converting operations, not only for economical reasonsbut also for the purpose of building in adequate structural integrity inthe processed web (fiber, film) during converting, where for examplecrystallization times are typically less than about 3 seconds oncommercial film and fiber lines.

In the past, the biodegradable and physical properties of a variety ofPHA's have been studied, and reported. Polyhydroxyalkanoates aregenerally semicrystalline, thermoplastic polyester compounds that caneither be produced by synthetic methods or by a variety ofmicroorganisms, such as bacteria and algae. The latter typically produceoptically pure materials. Traditionally known bacterial PHA's includeisotactic Poly(3-hydroxybutyrate), or i-PHB, the high-melting, highlycrystalline, very fragile/brittle, homopolymer of hydroxybutyric acid,and Poly(3-hydroxybutyrate-co-valerate), or i-PHBV, the somewhat lowercrystallinity and lower melting copolymer that nonetheless suffers thesame drawbacks of high crystallinity and fragility/brittleness. PHBVcopolymers are described in the Holmes et al U.S. Pat. Nos. 4,393,167and 4,880,59, and until recently were commercially available fromImperial Chemical Industries under the trade name BIOPOL. Their abilityto biodegrade readily in the presence of microorganisms has beendemonstrated in numerous instances. These two types of PHA's however areknown to be fragile polymers which tend to exhibit brittle fractureand/or tear easily under mechanical constraint. Their processability isalso quite problematic, since their high melting point requiresprocessing temperatures that contribute to their extensive thermaldegradation while in the melt. Finally, their rate of crystallization isnoticeably slower than traditional commercial polymers, making theirprocessing either impossible or cost-prohibitive on existing convertingequipment.

Other known PHA's are the so-called long side-chain PHA's, or isotacticPHO's (poly(hydroxyoctanoates)). These, unlike i-PHB or PHBV, arevirtually amorphous owing to the recurring pentyl and higher alkylside-chains that are regularly spaced along the backbone. When present,their crystalline fraction however has a very low melting point as wellas an extremely slow crystallization rate, two major drawbacks thatseriously limit their potential as useful thermoplastics for the type ofapplications mentioned in the field of the invention.

Recently, new poly(3-hydroxyalkanoate) copolymer compositions have beendisclosed by Kaneka (U.S. Pat. No. 5,292,860), Showa Denko (EP 440165A2,EP 466050A1), Mitsubishi (U.S. Pat. No. 4,876,331) and Procter & Gamble(U.S. Pat. Nos. 5,498,692; 5,536,564; 5,602,227; 5,685,756). Alldescribe various approaches of tailoring the crystallinity and meltingpoint of PHA's to any desirable lower value than in thehigh-crystallinity i-PHB or PHBV by randomly incorporating controlledamounts of “defects” along the backbone that partially impede thecrystallization process. Such “defects” are either, or a combination of,branches of different types (3-hydroxyhexanoate and higher) and shorter(3HP, 3-hydroxypropionate) or longer (4HB, 4-hydroxybutyrate) linearaliphatic flexible spacers. The results are semicrystalline copolymerstructures that can be tailored to melt in the typical use range between80° C. and 150° C. and that are less susceptible to thermal degradationduring processing. In addition, the biodegradation rate of these newcopolymers is typically accrued as a result of their lower crystallinityand the greater susceptibility to microorganisms. Yet, whereas themechanical properties and melt handling conditions of such copolymersare generally improved over that of i-PHB or PHBV, their rate ofcrystallization is characteristically slow, often slower than i-PHB andPHBV, as a result of the random incorporation of non-crystallizabledefects along the chains. Thus, it remains a considerable challenge toconvert these copolymers into various forms by conventional meltmethods, for they lack sufficient structural integrity or they remainsubstantially tacky, or both, after they are cooled down from the melt,and remain as such until sufficient crystallization sets in. Residualtack typically leads to material sticking to itself or to processingequipment, or both, and thereby can restrict the speed at which apolymeric product is produced or prevent the product from beingcollected in a form of suitable quality. Hence, significant improvementsin the rate of crystallization are needed if these more desirablecopolymers are to be converted into films, sheets, fibers, foams, moldedarticles, nonwoven fabrics and the like, under cost-effectiveconditions.The issue of the slow crystallization rate of PHBV is awell-recognized one and has been addressed previously either in the openliterature or in patent applications which disclose a variety of optionsthat can help enhance its crystallization rate.

For example, Herring et al.'s U.S. Pat. No. 5,061,743 discloses the useof a combination of an organophosphonic acid or ester compound and ametal oxide, hydroxide or carboxylate salt as nucleating agents toimprove the crystallization rates of PHA's such as PHB. It builds uponan earlier British composition patent by Binsbergen for crystallinelinear polyesters (GB 1,139,528). Similarly, Organ et al. in U.S. Pat.No. 5,281,649 discloses the use of ammonium chloride as a nucleatingagent to improve the crystallization rates of PHAs, for example PHB. Thesmall size of the nucleant minimizes problems of opacity andagglomeration otherwise experienced with particulates. Additionalexamples of additives blended with PHA's that improve theircrystallization rate can be found. For example, U.S. Pat. No. 5,516,565,to Matsumoto, proposes the use of crystallization agents such asaromatic aminoacids, e.g. tyrosine and phenyl alanine, that are capableof being decomposed or metabolized in an animal or in the environment,hence allowing the use of nucleated PHA in medical devices. In 1984, P.J. Barham wrote a review of the different types of nucleants in anarticle entitled “Nucleation behavior of poly-3-hydroxybutyrate” (J.Mater. Sci., 19, p. 3826 (1984)). He notes that the nucleating effect ofimpurities such as talc comes from their ability to reduce the entropyof partially adsorbed molecules, whereas additives such as saccharinwork by epitaxial, crystallographic matching. He also describedself-seeding, a phenomenon that produces an increase in the nucleationdensity of semicrystalline polymers, with however very limited practicalimplications since the polymer must be kept within only a few degrees ofthe peak melting point of the polymer. In a different article, Organ etal. also elucidate the epitaxial growth of PHB off ammonium chloridecrystals and demonstrated positive results with boron nitride, saccharinand the hydrogen-peroxide salt of urea as nucleating agents (J. Mater.Sci., 27, p. 3239 (1992)). Finally, Hobbs et al. report about thebeneficial effect of water on the crystal growth rate of thin films ofpoly(hydroxybutyrate) in a published article (Polymer, 38, p. 3879(1997)).

Blends containing PHA's are also disclosed with potential benefits ontheir crystallization rate, and several scientific studies have beenaimed at characterizing such blends. For instance, a Japanese patentassigned to Mitsubishi Rayon (JP Patent No. 63172762) reports on the useof i-PHB as an additive to PET in order to improve its crystallizationrate. Kleinke et al., in U.S. Pat. No. 5,231,148, teach about a mixturecontaining polyhydroxyalkanoate and compounds with reactive acid andalcohol groups which possesses better mechanical properties andcrystallizes at a higher temperature than the pure PHA. Hammonddiscloses polymer compositions containing a PHA polymer and an oligomerselected from the group: PHA's, polylactide, polycaprolactone andcopolymers thereof (U.S. Pat. No. 5,550,173). In World PatentApplication No. 96/09402, Cox et al. describe a hydroxycarboxylic acidcopolyester comprising non-random blocks of different compositions, thehigher melting component contributing to reduce the crystallization timeof the overall material. In their scientific article published inPolymer, 34, p. 459 (1993)), Organ et al. examine the phase behavior andthe crystallization kinetics of melt blends of i-PHB with PHBV (w/18.4%valerate) over their entire composition range, in 10% composition changeincrements. Their data indicate separate melt and two crystal phases inthe case of blends that contain a majority of the PHBV copolymer. Theauthors however fail to recognize and establish positive consequencesthat such blend structures may have on their crystallization rate. In ascientific study published in Makrom. Chem., Makrom. Symp., 19, p. 235(1988), Marchessault et al. describe the process of solution-blendingi-PHB with PHBV in chloroform, followed by their co-precipitation indiethyl ether. Horowitz et al. describe an in-vitro procedure forpreparing artificial granules made of i-PHB with PHO (using ultrasoniccentrifugation) which produces a single, uniform population of granulesthat retain their amorphous elastomeric state (Polymer, 35, p. 5079(1994)).

More immediately relevant to the present invention, Liggat in U.S. Pat.No. 5,693,389 discloses dry blending a higher melting PHA such as PHB inpowder form to serve as a nucleating agent for a lower melting PHA suchas PHBV. Although the idea has a positive impact on the crystallizationrate, the crystallization rate benefit is limited by the relativelylarge size and the low dispersibility of the PHB powder. In addition,the size of the dispersed PHB powder generally impedes processing ofsuch blends into thin products like films, coatings or fibers (due todie clogging), and can also be responsible for their low aesthetics andweakened mechanical properties (e.g., stress concentration loci in thefinal articles, opacity, etc.). Moreover, the close vicinity of thei-PHB and PHBV melting points is responsible for the limited size of theprocessing temperature window where the nucleating i-PHB particlesremain active. Very recently, Withey and Hay reinvestigated seedingphenonema and their influence on the crystallization rate in blends ofi-PHB and PHBV (Polymer, 40, p. 5147 (1999)). Their approach howeverfailed to generate better results for the use of i-PHB as a nucleatingagent over boron nitride.

Hence, all prior reported attempts to improve the crystallization ratesof PHA polymers and copolymers have been unsatisfactory in that thecrystallization rate remains too low for commercial processing, and thenucleating agent can disadvantageously affect one or more properties ofthe polymer or copolymer, for example rendering them opaque orintroducing loci of stress concentration, hence compromising thephysical and mechanical or biodegradable properties of the polymers.

In addition to the above methods of chemical modification or blending ofPHA's, there are also prior accounts of thermal treatment and specialhandling of PHA's that are said to contribute to increasing theircrystallization rate as well as improving their physical properties. Forinstance, in U.S. Pat. No. 4,537,738, Holmes describes a process ofpreforming a partially crystallized PHB extruded form before subjectingit to a drawing stage and allowing completion of the crystallization inthe stretched state. Waddington, in U.S. Pat. No. 5,578,382 proposes toachieve a high density of nucleation sites by cooling down a PHA filmjust above Tg (4-20° C.), before bringing the temperature back uptowards the optimum temperature for crystal growth, for the purpose ofachieving more rapid crystallization, smaller spherulites and improvedbarrier properties. De Koning et al. (Polymer, 34, p. 4089 (1993) &Polymer, 35, p .4599 (1994)) as well as Biddlestone et al. (Polym. Int.,39, p. 221 (1996)) studied the phenomena of physical aging andembrittlement in i-PHB or PHBV and attributed it to the occurrence ofsecondary crystallization with time. The phenomenon may be partiallyprevented or reversed by thermal annealing, by virtue of a change inmorphology and a reduction of the overall amorphous-crystallineinterface. De Koning (WO 94/17121) and Liggat et al. (WO 94/28047 and WO94/28049) suggest the use of a post-conversion heating treatment to atleast partially restore the mechanical properties of i-PHB or PHBV thatare affected by physical aging and which is responsible for theembrittlement of the material over time. The same approach is proposedby Liggat et al (WO 94/28048) for these materials in the presence of aplasticizer.

Most of these process conditions applied to i-PHB or PHBV however failto impart satisfactory physical and mechanical properties to thematerials which generally tend to remain fragile. Accordingly, it wouldbe advantageous to obtain PHA's which not only have improvedcrystallization rates, but also exhibit an advantageous combination ofphysical/mechanical properties allowing formation and use of shapedarticles that are useful in a wide range of applications.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the present invention to providesemicrystalline polyhydroxyalkanoate-containing compositions and methodsfor preparing such compositions which overcome disadvantages orlimitations of the prior art. It is a related object of the presentinvention to provide semicrystalline polyhydroxyalkanoate compositionscomprising biodegradable copolymers having improved crystallizationrates and process methods that provide shaped articles made out of suchcompositions. It is a further object of the invention to provide methodsfor improving the crystallization rates of semicrystallinepolyhydroxyalkanoates so that their conversion into shaped articles iseither enabled or improved using conventional converting processes suchas melt or solvent spinning, flash spinning, melt blowing, cast filmextrusion or blown film extrusion, extrusion blow molding, injectionmolding or solvent coating. It is a further object of the invention toprovide a biodegradable method for boosting the nucleation density, andas a result the overall crystallization rate, of biodegradablepolyhydroxyalkanoates.

It is an additional object of this invention to provide tough, strong,yet flexible biodegradable sanitary and medical garments, compostableplastic bags and agricultural films, injection-molded pots, yard-wastenets, compostable foamed articles, biodegradable pulp, paper coatings,binders and the like, made out of the compositions of the presentinvention.

It is yet a further object of the invention to provide methods forforming shaped products that comprise semicrystallinepolyhydroxyalkanoates with improved physical and mechanical properties.It is a further object of this invention to minimize physical aging andembrittlement of semicrystalline polyhydroxyalkanoates with time.

SUMMARY OF THE INVENTION

These and additional objects and advantages are provided by thecompositions, methods and shaped articles of the present invention. Inone embodiment, the invention is directed to compositions comprising atleast two polymer components:

(a) wherein the first component, which makes up the bulk of thecomposition, is a crystallizable biodegradable polyhydroxyalkanoatecopolymer, or a blend thereof, comprising at least two randomlyrepeating monomer units (RRMU's), wherein the first randomly repeatingmonomer unit, which comprises at least 50% of the totalpolyhydroxyalkanoate monomer units, has the structure (I):

 wherein R¹ is H, or C1 or C2 alkyl, and n is 1 or 2; and the secondrandomly repeating monomer unit included in the polyhydroxyalkanoatecopolymer is different from the first randomly repeating monomer unitand comprises at least one monomer selected from the group consisting ofthe structures (II) and (III):

 wherein R² is a C3-C19 alkyl or C3-C19 alkenyl, and

 wherein m is from 2 to about 16, wherein the polyhydroxyalkanoatecopolymer has a number average molecular weight of greater than about100,000 g/mole, and further wherein the first biodegradablepolyhydroxyalkanoate has a melting point Tm1, and:

(b) a second crystallizable biodegradable polyhydroxyalkanoatehomopolymer or copolymer, or a blend thereof, which is finely dispersedwithin the bulk of the first biodegradable polyhydroxyalkanoatecopolymer and which comprises at least one randomly repeating monomerunit having the structure (IV):

 wherein R³ is H, or C1 or C2 alkyl, and p is 1 or 2; Optionally, thesecond biodegradable polyhydroxyalkanoate polymer can further comprisetwo or more additional randomly repeating monomer units selected fromthe group consisting of the structures (V) and (VI):

 wherein R⁴ is a C2-C19 alkyl or C2-C19 alkenyl, and

 wherein q is from 2 to about 16, wherein the additional randomlyrepeating monomer units represent up to 25% of the total monomer units,wherein the second biodegradable polyhydroxyalkanoate polymer suitablyhas a number average molecular weight of greater than about 50,000g/mole, and further wherein the second biodegradablepolyhydroxyalkanoate has a melting point Tm2. The second PHA meltingpoint Tm2 is at least about 20° C. greater than the Tm1 of the firstPHA, i.e., Tm2≧(Tm1+20° C.).

The intimate dispersion of the second biodegradable polyhydroxyalkanoatepolymer (b) within the bulk of the first polyhydroxyalkanoate copolymer(a) is achieved by blending these two components in solution or in themelt, while in the presence of potential additional constituents. Thisnot only results in a blend structural composition with a highercrystallization rate, but also allows such a composition to be processedon standard fiber and film converting equipment

In another embodiment, the invention is directed to a method forenhancing the rate of crystallization of a first biodegradablepolyhydroxyalkanoate copolymer, or a blend thereof, comprising at leasttwo randomly repeating monomer units, wherein the first randomlyrepeating monomer unit has the structure (I):

wherein R¹ is H, or C1 or C2 alkyl, and n is 1 or 2; and the secondrandomly repeating monomer unit is different from the first randomlyrepeating monomer unit and comprises at least one monomer selected fromthe group consisting of the structures (II) and (III):

wherein R² is a C3-C19 alkyl or C3-C19 alkenyl, and

wherein m is from 2 to about 16, and wherein at least about 50 mole % ofthe copolymer comprises randomly repeating monomer units having thestructure of the first randomly repeating monomer unit (I), and furtherwherein the copolymer has a melting point Tm1. The method comprises astep of dispersing in the first biodegradable polyhydroxyalkanoatecomponent, at the molecular level, a second biodegradablepolyhydroxyalkanoate homo- or copolymer, or blend thereof, comprising atleast one randomly repeating monomer unit having the structure (IV):

wherein R³ is H, or C1 or C2 alkyl, and p is 1 or 2. Optionally, thesecond biodegradable polymer further comprises two or more randomlyrepeating monomer units selected from the group consisting of thestructures (V) and (VI):

wherein R⁴ is a C2-C19 alkyl or C2-C19 alkenyl, and

wherein q is from 2 to about 16, wherein the additional randomlyrepeating monomer units represent up to 25% of the total monomer units,wherein the second biodegradable polyhydroxyalkanoate polymer suitablyhas a number average molecular weight of greater than about 50,000g/mole, and further wherein the second biodegradablepolyhydroxyalkanoate has a melting point Tm2. The second PHA meltingpoint Tm2 is at least about 20° C. greater than that the Tm1 of thefirst PHA, i.e., Tm2≧(Tm1+20° C.). The fine dispersion is achieved byblending the two components in the melt, e.g. in a heated extruder, at atemperature above their respective melting points, or in solution, in acommon solvent.

In yet further embodiments, the invention is directed to methods forsuccessfully and efficiently converting the compositions of the presentinvention into shaped articles, such as films, fibers, nonwovens,coatings, injection moldings, blow moldings and the like, using standardprocessing equipment known to the field of polymer processing. Themethods encompass processing the compositions at a temperature selectedin the interval between Tm1 and Tm2, which spans a temperature range ofmore than 20° C. by virtue of the above relationship between Tm1 and Tm2stated above. Also, the methods encompass forming and crystallizing theshaped articles at an elevated temperature selected within 25° C. withinthe optimal crystallization temperature, i.e. in the range between about30° C. and 90° C., where crystal growth rate is maximized while takingadvantage of the extremely high nucleation density that is provided bythe compositions of the present invention. The resultant semicrystallinestructure exhibits improved resistance to physical aging andembrittlement that otherwise negatively affects the mechanicalproperties with time. It eliminates the need of annealing the productand therefore simplifies the overall process of making shaped articles.The invention also includes a variety of useful shaped articles andfinal products formed by such processing methods usingpolyhydroxyalkanoate compositions of the present invention. This includetough, strong and flexible biodegradable sanitary and medical garments,compostable plastic bags and agricultural films, injection-molded pots,yard-waste nets, compostable foamed articles, biodegradable pulp, papercoatings, binders and the like.

The compositions and the methods of the invention provide thepolyhydroxyalkanoate copolymer compositions with unsurpassedcrystallization rates and therefore facilitate the use ofpolyhydroxyalkanoate copolymers in the production of articles therefrom.In a final embodiment, the polyhydroxyalkanoate compositions may beblended with compatible polymers other than PHA's and improve theprocessability, crystallization rate and final physical/mechanicalproperties. The other blend components must be selected amongbiodegradable polymers in the blend compositions are to remainbiodegradable.

These and additional objects and advantages of the present inventionwill be more fully understood in view of the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description and examples will be more fullyunderstood in view of the drawing in which:

FIG. 1 sets forth a heat flow curve as a function of temperature forvarious compositions as described in Example 1;

DETAILED DESCRIPTION OF THE INVENTION

The compositions according to the invention comprise at least first andsecond biodegradable polyhydroxyalkanoate components. The firstbiodegradable polyhydroxyalkanoate comprises a copolymer, or a blendthereof, comprising at least two RRMUs. The first RRMU has the structure(I):

wherein R¹ is H, or C1 or C2 alkyl, and n is 1 or 2. In a preferredembodiment, R¹ is a methyl group (CH₃), whereby the first RRMU has thestructure:

wherein n is 1 or 2. In a further preferred embodiment of the firstRRMU, R¹ is methyl and n is 1, whereby the polyhydroxyalkanoatecopolymer comprises 3-hydroxybutyrate units.

The second RRMU included in the first biodegradable polyhydroxyalkanoatecopolymer comprises at least one monomer selected from the groupconsisting of the structures (II) and (III):

wherein R² is a C3-C19 alkyl or C3-C19 alkenyl, and

wherein m is from 2 to about 16. Generally, in the RRMU of formula (II),the length of R² will, to some extent, influence the reduction inoverall crystallinity of the copolymer.

In a preferred embodiment, R² is a C3-C10 alkyl group or alkenyl group.In a further preferred embodiment, R² is a C3-C6 alkyl group, and in afurther preferred embodiment, R² is a C3 alkyl group. In alternatelypreferred embodiments, R² is a C10-C19 alkyl or alkenyl group. Withreference to the second RRMU comprising a monomer of structure (III), ina preferred embodiment, m is from 2 to about 10, and more preferably isfrom about 4 to about 8. In a further preferred embodiment, m is about5. In further embodiments, the biodegradable polyhydroxyalkanoatecopolymer comprises the first RRMU of structure (I) and second RRMUs ofboth structure (II) and structure (III).

In order to obtain an advantageous combination of physical propertiesand biodegradability of the polyhydroxyalkanoate copolymer, at leastabout 50 mole % of the copolymer comprises RRMUs having the structure ofthe first RRMU of formula (I). Suitably, the molar ratio of the firstRRMUs to the second RRMUs in the copolymer is in the range of from about50:50 to about 99:1. More preferably, the molar ratio is in the range offrom about 75:25 to about 95:5, and even more preferred is in the rangeof from about 80:20 to about 95:5. In yet further preferred embodiments,the molar ratio of the first RRMUs to the second RRMUs is in the rangeof from about 85:15 to about 95:5. In addition, the polyhydroxyalkanoatecopolymer suitably has a number average molecular weight of greater thanabout 100,000 g/mole, and further wherein the first biodegradablepolyhydroxyalkanoate has a melting point Tm1. While not intending to bebound by theory, it is believed that the combination of the second RRMUchain and/or branch lengths and the indicated molar amounts sufficientlydecrease the crystallinity of the first RRMU to form the copolymer withdesired physical properties.

In further embodiments of the polyhydroxyalkanoate copolymer employed inthe compositions, one or more additional RRMUs may be included.Suitably, the additional RRMUs may have the structure (VII):

wherein R⁵ is H, or a C1-C19 alkyl or alkenyl group and s is 1 or 2,with the provision that the additional RRMUs are not the same as thefirst or second RRMUs. The compositions further comprise a secondbiodegradable polyhydroxyalkanoate homo- or copolymer, or blend thereof,comprising at least one randomly repeating monomer unit having thestructure (IV):

wherein R³ is H, or C1 or C2 alkyl, and p is 1 or 2. In a preferredembodiment, R³ is a methyl group (CH₃), whereby the RRMUfor the secondbiodegradable polyhdroxyalkanoate has the structure:

wherein p is 1 or 2. In a further preferred embodiment, R³ is methyl andp is 1, whereby the second polyhydroxyalkanoate polymer comprises3-hydroxybutyrate units. In a further preferred embodiment, the secondbiodegradable polymer is the polyhydroxybutyrate homopolymer.Optionally, the second biodegradable polymer comprises two or moreadditional randomly repeating monomer units selected from the groupconsisting of the structures (V) and (VI):

wherein R⁴ is a C2-C19 alkyl or C2-C19 alkenyl, and

wherein q is from 2 to about 16. With reference to the second RRMUcomprising a monomer of structure (VI), in a preferred embodiment, q isfrom 2 to about 10, and more preferably is from about 4 to about 8. In afurther preferred embodiment, q is about 5. When present, the additionalrandomly repeating monomer units represent no more than 25% of the totalmonomer units, preferably less than 15%, wherein the secondpolyhydroxyalkanoate homo- or copolymer suitably has a number averagemolecular weight of greater than about 50,000 g/mole, and furtherwherein the second biodegradable polyhydroxyalkanoate has a meltingpoint Tm2. The second biodegradable polyhydroxyalkanoate has a meltingpoint, Tm2, which is at least about 20° C. greater than the meltingpoint, Tm1, of the first biodegradable polyhydroxyalkanoate, so that theequation Tm2≧Tm1+20° C. is satisfied. The value of the melting point isgenerally determined by DSC (Differential Scanning Calorimetry) and istaken as the highest endothermic peak temperature observed on the DSCheating scan using, for example, the method outlined in ASTM D 3418.Although not intending to be bound by theory, it is believed that thesecond biodegradable polyhydroxyalkanoate can act as a nucleating agentfor the first biodegradable polyhydroxyalkanoate and thereby improve thecrystallization rate of the first biodegradable polyhydroxyalkanoate ifthe adequate blend composition, structure and high level of dispersionis achieved. In a more specific embodiment, the second PHA meltingpoint, Tm2, is at least about 25° C. greater than the melting point,Tm1, of the first PHA. In yet further embodiments, the second PHAmelting point, Tm2, is at least about 30° C. greater than the first PHAmelting point, Tm1, or the second PHA melting point, Tm2, is at leastabout 30° C. greater, but not more than about 60° C. greater, than thefirst PHA melting point, Tm1.

In accordance with an important aspect of the invention, the novelcompositions according to the invention are formed by solution blendingor melt blending of the first and second biodegradablepolyhydroxyalkanoates. It has been discovered that either solutionblending or melt blending of the first and second biodegradablepolyhydroxyalkanoates provides sufficient dispersion of the secondbiodegradable polyhydroxyalkanoate within the first biodegradablepolyhydroxyalkanoate for the second biodegradable polyhydroxyalkanoateto significantly improve the crystallization rate of the firstbiodegradable polyhydroxyalkanoate. As will be discussed in detail belowin the examples, an improvement in crystallization rate is evidenced bya reduction in the time required for the appearance of a crystallizationexotherm on a Differential Scanning Calorimetry (DSC) scan, upon coolingdown the composition from a given melt temperature.

A majority of the composition preferably comprises the firstbiodegradable polyhydroxyalkanoate, whereby the second biodegradablepolyhydroxyalkanoate is finely dispersed throughout a continuous phaseor matrix of the first component and is included in an amount sufficientto improve the crystallization rate of the first component. In oneembodiment, compositions comprise from about 0.01 to about 10 weightpercent of the second PHA component (b). In more specific embodiments,the compositions comprise from about 0.1 to about 5 weight percent ofthe second PHA component (b). In even more specific embodiments, thecompositions comprise from about 0.1 to about 3 weight percent of thesecond PHA component (b).

The biodegradable polyhydroxyalkanoate components included in thecompositions of the invention can be synthesized by synthetic chemicalor biological based methods as disclosed, for example, by Noda in U.S.Pat. No. Re. 36,548, which is incorporated herein by reference.

As set forth above, the compositions according to the present inventionwhich comprise the first and second PHA components are prepared bysolution blending or melt blending. In solution blending processes, bothcomponents are at least partially dissolved in a common solvent, forexample chloroform or acetone, although other solvents will be apparentto those skilled in the art. It will be appreciated that the second PHAcomponent may only partially solubilize in the common solvent, or mayfully solubilize in the common solvent, and both of these describedembodiments are within the scope of the present solution blendingmethods. It will also be appreciated that the second,higher-crystallinity and higher melting component may be selected to bein the amorphous state prior to be solubilized in order to improve itssolubility. This is easily achieved by quenching the polymer from themelt. Other methods include the ultrasonic emulsification of the polymerfor the preparation of artificial granules which retain their amorphousstate, as described by Horowitz et al (Polymer, 35, p. 5079 (1994)). Inthe case of only partial solubilization of the component (b), it ispreferable to filter out the non-soluble fraction. The resulting blendcompositions are allowed to crystallize together by any technique knownin the art, including, but not limited to, cooling of the solution,precipitation of the blended polymer components in a non-solvent, orevaporation of the common solvent. Additionally, two or more of thesecrystallization techniques may be combined if desired.

The solution blending methods according to the invention may also beachieved as an integral part of any solvent-based process for productionof the components, including, but not limited to, biomass separationprocesses, polymer extraction and the like used for the recovery of thefirst PHA component. As an example, an acetone-solubilized first PHAcomponent comprising a branched copolymer is combined with a partiallyacetone-solubilized, yet-to-crystallize amorphous second PHA componentin hot, or preferably cold, acetone. In further embodiments,bacterially-produced or transgenic-plant-produced PHA copolymersrepresenting the first PHA component may be combined in solution withthe second PHA component in its quenched form from a melt, in the formof crystallizable particles coated with a surfactant or phospholipid inorder to maintain its amorphous state, or the like.

Alternatively, the compositions according to the present invention maybe prepared by melt blending the first and second PHA components. Thetemperature of the melt should be greater than the melting point of thesecond, higher melting PHA component, and sufficient shear mixing shouldbe applied to ensure adequate dispersion of the second PHA componentwithin the matrix of the first PHA component. Sufficient shear mixingcan be obtained by many techniques known in the art, including but notlimited to, continuous mixing in a single- or twin-screw extruder orbatch mixing in a Banbury mixer. After melting and mixing, the blendedcompositions are allowed to crystallize by any technique known in theart, including but not limited to, quenching of the melt below its' melttemperature in a water bath or by air cooling. In addition, thecrystallization step can be carried out in the presence or absence ofshear or extensional flows, or in any combination of flow fieldsthereof. In a preferred embodiment, the second, higher-melting PHAcomponent may be plasticized or mixed with a miscible component, orboth, to achieve adequate dispersion at blending temperatures below themelting point of the neat higher-melting PHA, and therefore reduce therisk of thermal degradation and/or detrimental losses in molecularweight in the PHA components during blending. Suitable plasticizers orother miscible components will be apparent to those skilled in the artand include, but are not limited to, glycerol compounds, for exampleglycerol triacetate, polyalkylene oxides, for example polyethyleneoxide, cellulose esters, for example cellulose acetate propionate andcellulose acetate butyrate, chitan, chitosan and the like.

While not being bound by theory, it is believed that blending the highermelting PHA component with a plasticizer or a miscible component, orboth, can reduce the melting temperature or increase the percentage ofthe crystalline phase melted at temperatures below the peak melttemperature of the neat PHA (Tm2), or both. In either case at meltblending temperatures below Tm2, more of the modified higher meltingcomponent can be adequately dispersed within the matrix of the first orlower melting PHA component as compared to the same amount of the neathigher melting PHA. Additionally, while not being bound by example,Scandola, et al. (Macromolecules 1992, 25, 6441) and Buchanan, et al.(Macromolecules 1992, 25, 7373) show that the crystallinity of PHB andPHBV are completely depressed when blended with more than about 50weight percent cellulose acetate propionate or cellulose acetatebutyrate. That is, both PHB and PHBV are completely amorphous in thisstate, and therefore much more amenable to adequate dispersion withinthe matrix of a lower melting PHA at blending temperatures below themelting temperatures of neat PHB or neat PHBV. As a result of thesolution blending or melt blending of the first and second PHAcomponents as described herein, a blend composition having a unusualbroader melting endotherm that extends towards higher temperaturesresults. While not being bound by theory, the broader melting endothermis suggestive of a broader distribution of crystalline species that notonly encompasses the original melting range of the predominant lowermelting first PHA but extends well above it, over the temperature rangedelineated by the higher melting second PHA component when examined byDSC. For example, solution blending of the first and second PHAcomponents, followed by precipitation in a non-solvent, produces acomposition exhibiting a single, broad expanded melting endotherm, thehigh-temperature end of which is representative of an array ofintermediate melting crystalline entities having melting characteristicsranging between those of the first and second PHA components. In anotherembodiment, wherein solution blending of the first and second PHAcomponents is followed by a precipitation by solvent evaporation, theextended melting range may give rise to the observation of additionalmaxima in the melting endotherm for the blend over the temperature rangedefined by the melting point of the original components. On the otherhand, intermediate melting crystalline species are typically notobtained when the components are combined by dry blending, which is aconsequence of the much coarser dispersion, and the loss in nucleationefficiency.

The broad extension of the melting endotherm which is achieved in theblend compositions of the invention provides a wide temperature windowfor melt processing of such blends owing to the presence of an array ofresidual intermediate melting species that can initiate crystallizationduring subsequent converting and cooling. While not being bound bytheory, the high level of dispersion of the higher melting second PHAcomponent in the crystalline phase of the lower melting first PHAcomponent is believed to result in the significant improvements incrystallization rates obtained by the present invention.

In one embodiment, the weight ratio of the first PHA copolymer blendedto the second PHA polymer comprises from about 99.9:1 to about 9:1, morepreferably the weight ratio is from 99:1 to about 19:1 weight percent,and even more preferred is the range of 99:1 to about 32:1.

The compositions preferably comprise greater than about 50 weightpercent of the first polyhydroxyalkanoate copolymer. In one embodiment,the composition may comprise the first and second polyhydroxyalkanoatepolymers as the only polymeric components, while in yet otherembodiments, one or more additional polymers or copolymers may beincluded in combination with the first and second polyhydroxyalkanoatepolymers. For example, the compositions may include additionalbiodegradable polyhydroxyalkanoate polymers or copolymers, and/oradditional polymeric components, for example additional polyestercomponents or the like. In such embodiments, the biodegradable first andsecond polyhydroxyalkanoate components preferably comprise at leastabout 50 weight percent, more preferably at least about 60 weightpercent, and even more preferably at least about 75 weight percent, ofthe polymeric components of the compositions.

The compositions may further include various nonpolymeric componentsincluding, among others, antiblock agents, antistatic agents, slipagents, pro-heat stabilizers, antioxidants, pro- or antioxidantadditives, pigments, fillers and the like. Additionally, one or moreplasticizers may be employed in the compositions in conventionalamounts. A method for adding a plasticizer may for instance comprisemixing the higher-melting PHA component (b) with the plasticizer, forthe purpose of depressing its melting point or for increasing thepercentage of the crystalline phase melted at the blending temperature,or both, prior to melt-blending it with an unplasticized PHA component(a). The plasticizer then becomes a plasticizer for the final blendcomposition.

The compositions of the invention are suitable for forming shapedarticles and owing to the improved crystallization rates of thecompositions, are particularly advantageous for use in commercialprocessing applications. One skilled in the art will appreciate that thecompositions of the invention are suitable for use in preparing shapedarticles, such as fibers, nonwovens, films, coatings or moldings, andfor use in shaping processes including fiber spinning, film casting,film blowing, blow molding and injection molding. These processingtechniques are well known in the art and further detail herein is notrequired in order to enable one skilled in the art to use thecompositions of the present invention in such methods. One skilled inthe art will appreciate that the shaping processes will beadvantageously conducted at a temperature greater than Tm1 and at atemperature less than Tm2 in order to obtain the benefit of the improvedcrystallization rate of the compositions of the invention. In apreferred embodiment, the shaped processing is conducted at atemperature of from about 10 to about 30° C. greater than Tm1. Moreover,it will be appreciated that the selection of an optimal crystallizationtemperature Tc in the downstream process where shaped articles areformed will result in shorter solidification times and polymer formswhich exhibit reduced sensitivity to aging, i.e., stiffening and/orembrittlement. Preferably, the crystallization temperature Tc is in therange of about 20-90° C., more preferably in the range of about 30-80°C. whereby the resulting semicrystalline structure and morphologyexhibit surprisingly good resistance to physical aging and/or secondarycrystallization. While not being bound by theory, the compositions andarticles of the invention are also believed to exhibit a finerspherulitic morphology as well as thicker lamellar crystals resultingfrom the combination of the increased nucleation density and optimal,more thermodynamically favorable lamellar crystal growth conditions andthis resultant morphology is believed to provide improved mechanicalproperties to the compositions, with particular emphasis on ductilityand toughness. Additionally, unlike many previous compositionscontaining conventional nucleants, shape articles formed from thecompositions according to the invention exhibit good claritysubstantially comparable to that of the first PHA component alone.

All publications mentioned hereinabove are hereby incorporated in theirentirety by reference.

The compositions and methods of the present invention are furtherexemplified in the following examples. In the examples and throughoutthe present specification, parts and percentages are by weight unlessotherwise specified.

Differential Scanning Calorimetry (DSC) measurements are performedaccording to ASTM D 3418, where DSC samples are prepared by firstcompression molding a PHA composition into a thin film of around 0.003inches at about 140° C. between teflon sheets. The film is annealedovernight in a vacuum oven, with vacuum drawn, at a temperature of about65° C. Samples are punched out of the resulting films using a 6millimeter diameter skin biopsy punch. The samples are massed toapproximately 5-10 milligrams, loaded into small aluminum pans with lids(Perkin Elmer #0219-0041), and crimped using a Perkin Elmer StandardSample Pan Crimper Press (#0219-0048). Thermal tests and subsequentanalyses are performed using a Perkin Elmer DSC 7 equipped with PerkinElmer Thermal Analyses Software version 4.00.

The melt temperature of a PHA composition is determined by first heatingthe DSC sample from about 25° C. to 180° C. at a rate of 20° C. perminute and holding the sample at 180° C. for 3 minutes. The sample isthen quenched to minus 60° C. at a rate of 300° C. per minute, held for3 minutes at minus 60° C., then heated at a rate of 20° C. per minute to180° C. The melt temperature is taken as the highest peak temperature inthe second heat. If no melting peak is present in the second heat butthere is one in the first heat (which can happen for PHA compositionsthat crystallize very slow), the sample pan is removed from the DSC,allowed to remain at around 25° C. for 24 hours, reheated in the DSCfrom about 25° C. to 180° C. at a rate of 20° C. per minute, and thenthe melt temperature is taken as the highest peak temperature in thisthird heat.

The rate of crystallization of a PHA composition at a givencrystallization temperature is determined by first heating the DSCsample to the desired set temperature (which is above the melttemperature of the lower melting PHA), holding the sample at the settemperature for 2 minutes, and then subjecting the sample to a rapidcooling down to the desired crystallization temperature (about 300° C.per minute). As the temperature is held steady at the crystallizationtemperature, the crystallization process is evidenced by the appearanceof a crystallization exotherm in the DSC isothermal scan as a functionof time. A single-point characterization of the crystallization rateconsists of reporting the time at which the minimum in the exothermoccurs. The latter is often considered by those skilled in the art as areasonable indication of the half-time crystallization (t½) for thematerial.

EXAMPLE 1

The present example demonstrates solution blended compositions andmethods of the invention. The compositions comprise first and second PHAcomponents. The first PHA component is a copolymer of 3-hydroxybutyrate(RRMU of formula (I) wherein R² is OH₃ and n=1) and about 6.1 molepercent 3-hydroxyhexanoate (RRMU of formula (II) wherein R² is C3),abbreviated as PHBHx copolymer. The second PHA component is isotacticpolyhydroxybutyrate (i-PH B). Compositions 1A-1E are prepared asfollows: (1A) solution-blending of the PHBHx copolymer and about 2.0weight percent i-PHB in hot chloroform (50° C.), followed by solventevaporation; (1B) solution-blending of the PHBHx copolymer and about 2.0weight percent i-PHB in hot chloroform, followed by precipitation of thepolymer out of the solution with chilled methanol; (1C) dry-blending ofthe PHBHx copolymer and about 2.0 weight percent i-PHB bymixing/grinding the powders in the presence of dry ice; (1D) masterbatchof solution-blended PHBHx copolymer containing about 15% weight percenti-PHB (prepared in hot chloroform), which is then dry-blended withvirgin PHBHx; and (1E) solution-blending of the PHBHx copolymer with 1weight percent boron nitride, a conventional nucleating agent. Forcomparative purposes, a sample of the virgin PHBHx copolymer(composition 1F) is also prepared. Compositions 1A, 1B, 1C, and 1D areaccording to the invention while compositions 1E and 1F are forcomparison purposes. Using the Differential Scanning Calorimetry (DSC)technique described above to assess the rate of crystallization, thedata set forth in Table I illustrate the rate of crystallization ofcompositions 1A-1F for a given optimal crystallization temperature(56.3° C.), over a range of selected set temperatures prior to cooling.The half-time is the calorimetrically measured time it takes to reachabout ½ full crystallinity, and the set temperature is the temperatureat which the copolymer composition is equilibrated prior to beingquenched to the crystallization temperature.

TABLE 1 Crystallization half-time values for various PHA copolymercompositions Set Temp. (° C.) 130 140 150 160 170 Poly(3HB-co-3HX(6.1%)) + 6 6 7 47 133 2% i-PHB/via solution- blending +evaporation --- 1A Poly (3HB-co-3HX(6.1%)) + 7 8 8 69 171 2% i-PHB/viasolution- blending + precipitation --- 1B Poly (3HB-co-3HX(6.1%)) + 1831.5 71 120 129 2% i-PHB/via grinding + dry- blending --- 1C Poly(3HB-co-3HX(6.1%)) + 19 32 69 138 196 2% i-PHB/sol. masterbatch +dry-blending --- 1D Poly (3HB-co-3HX(6.1%)) + 18 30 65 116 131 1% boronnitride --- 1E Neat Poly (3HB-co- 24 36 84 168 220 3HX(6.1%)) --- 1F

As evidenced by Table I, there is a rapid, consistent increase in t½ incomparative compositions 1C-1F when the set temperature is raised fromthe original melting temperature of the PHBHx copolymer (Tm˜127° C.),even in the dry blended composition 1C and the conventionally nucleatedcomposition 1E. On the other hand, compositions 1A and 1B [according tothe invention] exhibit very steadily low t½ values up to greater thanabout 150° C., i.e. more than 20° C. above the melting point of theoriginal PHBHx copolymer. Hence, for these two systems, there is aprocessing temperature window of more than 20° C. above the originalmelting temperature of the PHBHx copolymer (Tm1) where the half time forcrystallization remains very low, i.e., below the measurable limit ofthe DSC method of ˜5 sec. At higher melt temperatures and up to theoriginal melting temperature of i-PHB (Tm2), compositions 1A and 1Bcontaining solution-blended i-PHB continue to outperform the otherblends, even though t½ values are seen to progressively increase.

To further illustrate the solution-blended compositions of the inventionand the dry-blended compositions, first scan heating isotherms forcompositions 1A and 1C are recorded from 25 to 190° C., the results ofwhich are set forth in FIG. 1. In the case of dry-blending, composition1C, two well distinguished and separate melting endotherms are observedand are characteristic of the two separate components of the blends. Onthe other hand, in the case of solution-blending, composition 1A, abroadening of the PHBHx melting endotherm, combined with the appearanceof intermediate melting species in the form of an expanded tail on thehigh-temperature side of the PHBHx, are observed and are indicative ofstructural changes in the blend. The temperature range defined by thehigh-temperature tail of the endotherm of composition 1A defines thepreferred processing window over which the high nucleation benefit isobserved.

EXAMPLE 2

In this example, a micro-extruder blown-film is prepared using acomposition comprising a PHBHx copolymer nucleated with 2%solution-blended i-PHB. More specifically, a micro-extruder blown filmset-up is used to assess the ability of the extruded polymer tocrystallize over short time scales. A 100 g batch of the composition 1Aas described in Example 1 is used (PHBHx copolymer which contains 2% ofsolution-blended i-PHB as a nucleating agent). Hot air is blown over thespace located above the film blowing die in order to reach a highercrystallization temperature Tc and cool down the film undermostfavorable conditions for both crystallization rate and physicalproperties. A video camera is used to record the progress of theexperiment. During the trial, the extruded polymer is seen to be capableof forming a tube which at times can expand into a bubble of yet limitedstability. At a melt-extrusion temperature of 160° C. and above, theextruded molten polymer remains largely amorphous and sticky. However,lowering the temperature of the melt down to about 150-155° C. producesthe appearance of a “frost line” a few inches above the die, indicativethat crystallization is already well underway a few seconds after thepolymer has come out. Stickiness is largely subdued and the tube shapeof the polymer remains stable. Thus, at a laboratory scale, themicro-extruded blown film further evidences the fast nucleation rate forthe solution-blended composition.

EXAMPLE 3

This example demonstrates the enhanced crystallization exhibited bycompositions prepared by solution blending methods employing partialsolubilization of amorphous i-PHB in acetone, a green solvent preferredfor PHA extraction.

More specifically, a copolymer of 3-hydroxybutyrate (3-HB) and about 8.4mole percent 3-hydroxyoctanoate (3-HO), abbreviated as PHBO copolymer,is first solubilized in hot acetone (at 3% concentration of polymer). Amelt-quenched amorphous i-PHB film sample is then added to the solution.The solution is either ice-chilled (composition 3B) or boiling-hot(composition 3A). Although the PHB film does not disappear totally, itbreaks down into small pieces and is indicative of its partialsolubility. To determine the crystallization rate improvement, samplesare taken out of the solution and allowed to dry and isothermalcrystallization scans are performed by DSC as described in Example 1. Asample of PHBO copolymer, without i-PHB or other nucleating agent,composition 3C, is also examined. The results are set forth in Table II.The data set forth in Table II demonstrates a large improvement in thecrystallization rate (a significant drop in t½ values) for thecompositions 3A and 3B that include i-PHB.

TABLE II Crystallization half-time values for various PHA copolymercompositions Set Temp. (° C.) 145 155 165 175 Poly (3HB-co-3HO(8.4%)) +16 26 205 too high i-PHB/via solution-blending in hot acetone + precip.--- 3A Poly (3HB-co-3HO(8.4%)) + 12 24 53 147 i-PHB/viasolution-blending ---3B in cold acetone + precip. Poly(3HB-co-3HO(8.4%)) --- 3C 65 280 too high too high

EXAMPLE 4

This example demonstrates the enhanced crystallization rate ofcompositions prepared by melt blending. More specifically, compositions4A-4C are prepared from compositions comprising a copolymer of3-hydroxybutyrate and about 6.7 mole percent 3-hydroxyhexanoate (PHBHxcopolymer) and 1.0 weight percent i-PHB, using three different blendingmethods: (4A) solution blending of the PHBHx copolymer and i-PHB inchloroform, followed by solvent evaporation, (4B) melt blending 500 mgof the two materials in a Mini Max Molder (Custom Scientific Instrumentsmodel CS-183-078, Whippany, N.J.) for 5 minutes at 160° C. (a mixingtemperature that is below the melt temperature of the i-PHB), afterwhich the sample is removed and allowed to cool, and (4C) the sameprocedure as (3B), but using a 180° C. mixing temperature (a mixingtemperature that is above the melt temperature of the i-PHB). Forfurther comparison, composition 4D comprising only PHBHx copolymer,without i-PHB or other nucleating agent, is prepared.

As in Example 1, Differential scanning analysis of the resulting blendsis performed to determine crystallization half-times where the half-timeis the calorimetrically measured time it takes to reach about one-halffull crystallinity (as determined by the minimum of the exotherm), andthe set temperature is the temperature to which the polymer blend istaken and held in the DSC, prior to being quenched to thecrystallization temperature Tc of 65° C. (a temperature at or near theoptimal crystallization temperature for this system).

The results are set forth in Table III:

TABLE III Crystallization half-time Set Temperature Sample BlendingProcedure 140° C. 180° C. 4D control (no i-PHB) 0.6 min 15.0 min 4Asolution blending <0.07 min 2.5 min (chloroform) (below DSC limits) 4Bmelt blending at 160° C. 0.3 min 4.7 min 4C melt blending at 180° C.<0.07 min 2.6 min (below DSC limits)

Solution blending and melt blending above the i-PHB melting point,compositions 4A and 4C, impart crystallization rates, respectively,which are faster than the lower limit attainable by DSC at a settemperature of 140° C. and which are significantly improved over thecontrol composition 4D. Melt blending below the i-PHB melt temperature(composition 4B), as described by the method preconized by Liggat'spatent, results in only a modest reduction compared to the controlcomposition 4D, and again highlight the benefits of the presentinvention. In addition, at the higher set temperature of 180° C.,compositions 4A and 4C similarly exhibit substantial improvement overthe controls.

EXAMPLE 5

This example demonstrates the enhanced crystallization rate ofcompositions prepared by melt blending a PHBO copolymer with plasticizedi-PHB. More specifically, a master batch of i-PHB plasticized withglycerol triacetate (GTA) is prepared by first solution blending the twocomponents in chloroform at a weight ratio of about 60:40 i-PHB:GTA, andthen allowing the blend to dry by evaporation. A copolymer of3-hydroxybutyrate and about 7.8 mole percent 3-hydroxyoctanoate (PHBOcopolymer) is melt blended with about 1.7 weight percent of thei-PHB/GTA masterbatch (yielding about 1.0 wt % i-PHB overall) in a MiniMax Molder (Custom Scientific Instruments model CS-183-078, Whippany,N.J.), where a total of 500 mg of PHBO and i-PHB/GTA are added to themixing chamber. The temperature is held constant at 160° C., which inthe case of the i-PHB/GTA masterbatch is above its melting point, andallows complete dispersion of the masterbatch in the PHBO copolymer.Indeed, in another series of experiment, we have shown that it ispossible to depress the melting point of i-PHB by 35° C., when blendedwith 50% of glycerol triacetate, or by 55° C., in the case of a blendcontaining 90% of the plasticizer. After a 5 minute mixing period, thesample, composition 5A, is removed and allowed to cool. Two additionalcompositions 5B and 5C are also prepared: (5B) melt blending the PHBOwith 1.0 wt % neat i-PHB for 5 minutes at 160° C.—in this case, thei-PHB is not melted and the method is reminiscent of that disclosed byLiggat; And (5C) melt mixing the PHBO for 5 minutes at 160° C. (nosecond PHA or other nucleator added)—as a control material in ourexperiment.

Analysis by Differential Scanning Calorimetry of the resulting blends isperformed to determine crystallization half-times as described inExample 4. The results are set forth in Table IV:

TABLE IV Crystallization half-times Set Temperature Sample NucleationSystem 140° C. 180° C. 5C none 1.7 min 12.0 min 5A 60:40 iPHB:GTA (1.0 w<0.07 min 4.8 min i-PHB) (below DSC limits) 5B neat i-PHB (1.0 wt %) 0.3min 7.1 min

At both set temperatures, but preferably at the lower set temperaturevalue, there are distinct improvements in the rate of crystallizationusing the plasticized i-PHB (composition 5A) as compared with the neati-PHB (composition 5B). The latter method (Liggat's composition andmethod) only yields a modest improvement over the virgin copolymer.

EXAMPLE 6

This example demonstrates the enhanced crystallization rate ofcompositions prepared by melt blending a PHBO copolymer with a miscibleblend of i-PHB and PEO. More specifically, a master batch of i-PHB andpoly(ethylene oxide) (PEO, average molecular weight of about 200) isprepared by first solution blending the two components in chloroform ata weight ratio of about 60:40 i-PHB:PEO, and then allowing the blend todry by evaporation. The PHBO copolymer from Example 5 is melt blendedwith about 1.7 weight percent of the i-PHB/PEO masterbatch (yieldingabout 1.0 wt % i-PHB overall) as described in Example 5 to providecomposition 6A. Two additional compositions 6B and 6C are also prepared:(6B) melt blending the PHBO with about 1.0 wt % neat i-PUB for 5 minutesat 160° C., and (6C) melt mixing the PHBO for 5 minutes at 160° C. (noi-PHB or other nucleator added).

Differential scanning analysis of the resulting blends is performed asdescribed in Example 4 to determine crystallization half-times. Theresults are set forth in Table V:

TABLE V Crystallization half-times Set Temperature Sample NucleationSystem 140° C. 180° C. 6C none 1.7 min 12.0 min 6A 60:40 iPHB:PEO (1.0wt % <0.07 min 5.3 min i-PHB) (below DSC limits) 6B neat i-PHB (1.0 wt%) 0.3 min 7.1 min

At both set temperatures, there are distinct improvements in the rate ofcrystallization using the miscible blend of i-PHB and PEO (composition6A) as compared with the neat i-PHB (composition 6B), and using thei-PHB-containing compositions (6A and 6B) as compared with the PHBOcomposition alone (6C).

EXAMPLE 7

This example demonstrates the improvement in physical properties that isaccompanied when higher crystallization temperatures are selected whileforming articles using the PHA compositions of the present invention.The focus here is on thoughness measurements which provide an indicationof the robustness of the films being tested. The so-called “biaxial teartest” is used to evaluate both stiffness and toughness properties ofexperimental films. The test consists of tensile loading a 3-inch wideby 0.5 inch long strip of film along its longer edges, using an Instron®universal testing machine, after a 1 inch-long pre-cut is placed in thecenter of the specimen using a sharp razor blade. For those acquaintedto the phenomenon of fracture, the mode of loading applied at the tip ofthe pre-existing cut is the cleavage mode, or tensile-opening mode,known as Mode I. The load experienced by the film upon drawing isrecorded by the load cell to construct the load-displacement curvecharacteristic of the material. From the above experimental curve, it ispossible to derive measures of both stiffness and toughness of thefilms, for the selected test conditions.

The initial linear rise in load provides a measure of the elasticproperty of the specimen ligaments prior to any failure initiation orgrowth. The maximum slope determined on the stress-strain curve providesa quantitative value of the elastic modulus:

E=∂σ/∂ε

The mechanical energy absorbed or dissipated in the specimen normalizedby its section defines the material's toughness and is experimentallyprovided by integrating the area under the curve. Three partial energyvalues are recorded: at the maximum load (σ_(max.)) of the recordedload-displacement curve, the load at ⅔σ_(max.), which describes thepoint where ⅓ of the mechanical integrity of the ligament is lost, andfinally ⅓ σ_(max.), at which point the ligament has lost ⅔ of itsmechanical integrity. For sake of simplicity and practicality in ourbenchmarking effort, the normalized partial energy up to a loss of ⅓mechanical integrity was chosen as a single-point characterization ofthe material toughness. T_(2/3) = ∫₀^(2/3  σ_(max))σ ⋅ ɛ

First, two random copolymers with 3-hydroxyhexanoic acid (3HX) atdifferent comonomer level (i.e. C₃H₇ branching) were examined. Films ofa synthetic poly(3HB-co-3HX(6.8%)) copolymer of high MW (˜685K) weremelt-pressed at 165° C. using a Carver Press and a three-step procedurenecessary to ensure the good quality of the melt-pressed films. Thesewere tested after being crystallized at two different temperatures (23°C. and 95° C.). Specimens crystallized at R.T. were found to exhibit lowtoughness and were virtually brittle, whereas those crystallized at 95°C. exhibited pseudo-ductile behavior and were 2 to 3 times tougher, atcomparable stiffness.

Similarly, films made of poly(3HB-co-3HX(10.8%)) which had been pressedat 155° C. and crystallized at either R.T., or 78° C. appeared to beslightly less stiff (˜330 MPa) and more than 40% tougher.

Another important consideration regarding PHA copolymers is whether thedetrimental effect of physical aging on their mechanical properties canbe minimized by high temperature crystallization, which in turn ispromoted by faster crystallization in a continuous process. For thatpurpose, a series of film specimens made of poly(3HB-co-8.4%3HO)) weretested either a couple of days after been pressed, or allowed to age atRT for ˜120 days prior to being tested. Pressing temperature was variedfrom R.T to 50° C. and 80° C. The resultant data entered in table VIreveals a general slight stiffening of the specimens (modulus ˜370 MPa)along with a significant loss in toughness (˜20 kJ/m²), raisingpotential concerns for the long-term physical integrity of thesematerials. Moreover, films crystallized at R.T. seem to undergo thelargest extent of stiffening (>400 MPa) along with a greater loss intoughness that actually led to their embrittlement, as a result ofaging. The data clearly support our finding that reducing the extent ofphysical aging in PHA's may be achieved by means of crystallizing thepolymer at high temperatures, which in turn can be promoted by theaddition of a nucleating agent that promotes faster crystallization inan actual process.

TABLE VI Mechanical Properties of PHA Copolymer Films Film PreparationStiffness Toughness PHA Copolymer Type Conditions (MPa) (kJ/m{circumflexover ( )}2) Poly(3HB-co-3HX(6.8%)) Crystallized @ 23C 485 8.5Poly(3HB-co-3HX(6.8%)) Crystallized @ 95C 495 21 Poly(3HB-co-3HX(10.8%))Crystallized @ 23C 310 42.5 Poly(3HB-co-3HX(10.8%)) Crystallized @ 78C350 59 Poly(3HB-co-3HO(8.4%)) Crystallized @ 23C 380 34Poly(3HB-co-3HO(8.4%)) Crystallized @ 50C 365 33 Poly(3HB-co-3HO(8.4%))Crystallized @ 80C 330 46 Poly(3HB-co-3HO(8.4%)) Crystallized @ 23C, 4208 aged Poly(3HB-co-3HO(8.4%)) Crystallized @ 50C, 350 18.5 agedPoly(3HB-co-3HO(8.4%)) Crystallized @ 80C, 370 28 aged

The specific embodiments and examples set forth above are provided forillustrative purposes only and are not intended to limit the scope ofthe following claims Additional embodiments of the invention andadvantages provided thereby will be apparent to one of ordinary skill inthe art and are within the scope of the claims.

What is claimed is:
 1. A composition, comprising (a) a continuous phaseof a first biodegradable polyhydroxyalkanoate comprising a copolymer ofat least two randomly repeating monomer units, wherein the firstrandomly repeating monomer unit has the structure (I):

wherein R¹ is H, or C1 or C2 alkyl, and n is 1 or 2; and the secondrandomly repeating monomer unit is different from the first randomlyrepeating monomer unit and comprises at least one monomer selected fromthe group consisting of the structures (II) and (III):

wherein R² is a C3-C19 alkyl or C3-C19 alkenyl, and

wherein m is from 2 to about 16, and wherein at least about 50 mole % ofthe copolymer comprises randomly repeating monomer units having thestructure of the first randomly repeating monomer unit (I), wherein thefirst biodegradable polyhydroxyalkanoate has a melting point Tm1: and(b) a second crystallizable biodegradable polyhydroxyalkanoatecomprising a randomly repeating monomer unit having the structure (IV):

wherein R³ is H, or C1 or C2 alkyl, and p is 1 or 2, wherein the secondbiodegradable polyhydroxyalkanoate has a melting point Tm2, wherein Tm2is at least about 20° C. greater than Tm1, and wherein the secondbiodegradable polyhydroxyalkanoate polymer (b) is finely dispersedwithin the bulk of the first biodegradable polyhydroxyalkanoate (a), toform a composition with enhanced crystallization and physicalproperties.
 2. A composition according to claim 1, wherein thecomposition is formed by solution blending of the components (a) and(b).
 3. A composition according to claim 1, wherein the composition isformed by melt blending of the components (a) and (b).
 4. A compositionaccording to claim 1, wherein the composition comprises from about 0.01to about 10 weight percent of component (b).
 5. A composition accordingto claim 4, wherein the composition comprises from about 0.1 to about 5weight percent of component (b).
 6. A composition according to claim 5,wherein the composition comprises from about 0.1 to about 3 weightpercent of component (b).
 7. A composition according to claim 1, whereinTm2 is at least about 25° C. greater than Tm1.
 8. A compositionaccording to claim 7, wherein Tm2 is at least about 30° C. greater thanTm1.
 9. A composition according to claim 7, wherein Tm2 is not more thanabout 60° C. greater than Tm1.
 10. A composition according to claim 1,wherein the first randomly repeating monomer unit of component (a) hasthe structure:

wherein n is 1 or 2, and the second randomly repeating monomer unit ofcomponent (a) has the structure:

wherein R² is a C3-C19 alkyl.
 11. A composition according to claim 10,wherein n is
 1. 12. A composition according to claim 11, wherein R² is aC3 alkyl.
 13. A composition according to claim 1, wherein the firstrandomly repeating monomer unit of component (a) has the structure:

wherein n is 1 or 2, and the second randomly repeating monomer unit ofcomponent (a) has the structure:

wherein m is from 2 to about
 16. 14. A composition according to claim13, wherein m is
 5. 15. A composition according to claim 14, wherein nis
 1. 16. A composition according to claim 1, wherein the molar ratio ofthe first randomly repeating monomer units to the second randomlyrepeating monomer units in component (a) is in the range of from about50:50 to about 99:1.
 17. A composition according to claim 16, whereinthe molar ratio of the first randomly repeating monomer units to thesecond randomly repeating monomer units in component (a) is in the rangeof from about 75:25 to about 95:5.
 18. A composition according to claim1, wherein component (a) has a number average molecular weight ofgreater than about 100,000 g/mole and wherein component (b) has a numberaverage molecular weight of greater than about 50,000 g/mole.
 19. Acomposition according to claim 1, wherein component (b) comprisesrepeating monomer units having the structure:

wherein p is 1 or
 2. 20. A composition according to claim 1, whereincomponent (b) includes a plasticizer.